1. Field of the Invention
The present invention relates to a high bulk density carbon fiber structure comprising a carbon fiber of optically anisotropic pitch as the main component, and to a process for producing the same.
More particularly, the present invention relates to a high bulk density carbon fiber structure and a process for producing the same, in which utilization of the large elongation and shrinkage of a phenolic resin fiber in combination with an optically anisotropic pitch fiber produces a carbon fiber structure comprising a carbon fiber of optically anisotropic pitch as the main component and increased in bulk density, while improving the handleability of sheets of the phenolic resin fiber and the pitch fiber, which may be either blended with each other in sheets or constitute respective sheets, as we11 as stabilizing a laminate structure of the sheets formed through entanglement thereof.
2. Prior Art
Carbon fiber structures have heretofore been produced by mutual superposition and adhesion of necessary pieces of a carbon fiber sheet material in a thinly and flatly spread form of fiber, such as a mat, a paper-like material, a non-woven fabric, a woven fabric, or a knitted fabric.
The kind of adhesive used in the production of such a carbon fiber structure greatly differs from one use application of the structure to another. For example, where the carbon fiber structure is used together with a plastic material to form a composite material, a choice is made of an adhesive having a good affinity for the plastic material and is occasionally having to be made of an adhesive which is substantially the same resin as the plastic material. Where the carbon fiber structure is used together with a metallic material to form a composite material, a choice is often desired to be made of an adhesive which is decomposed into vaporizable matter during the course of production of the composite material.
Where the carbon fiber structure is used together with a carbon material to form a composite material, a choice is desired to be made of an adhesive high in yield of carbon in carbonization thereof and capable of forming a high strength carbon material.
Carbon fibers have recently been increasingly evaluated as fibers for use in industrial materials. With an eye to such use of carbon fibers, there has been an increasing demand for a carbon fiber structure free of foreign matter and contaminants.
Taking as one example of industrial materials heat-insulating materials to be used at high temperatures, fibrous materials have heretofore been frequently used as such heat-insulating materials, while foamed materials have not so often been used. In the case of porous ceramic materials to be used as heat-insulating materials at high temperatures, the presence of any pores confined inside the materials entails a danger of explosive destruction of the materials when the internal pressure of the pores varies in keeping with a temperature change. Accordingly, open-cell porous ceramic materials as well as fibrous ones are evaluated to be better.
Under conditions necessitating a highly heat-resistant material, a high level of heat resistance inherent in carbon is highly evaluated. Since processed carbon materials which have heretofore been used as heat-insulating materials are mostly produced from natural materials as starting materials, however, they involve inherent problems, including contamination of themselves and surroundings with impurities derived from the natural materials.
The use of a carbon fiber high in purity of carbon may naturally be considered an effective solution to the problem of contamination. When an adhesive is used in the course of production of a processed carbon material in the form of a carbon fiber structure, however, the adhesive becomes a source of contaminants in most cases.
When an adhesive capable of complete decomposition upon heating is used to produce a processed carbon material in the form of a carbon fiber structure, the decomposition of the adhesive causes destruction of the processed carbon material.
An effective means as a solution to the foregoing problems may involve elimination of adhesives. From such considerations, interfiber entanglement has heretofore been a solution to the same problems with organic fibers.
Carbon fibers of optically anisotropic pitch type are very useful by virtue of their high strengths and high moduli of elasticity. However, they involve an essential difficulty in interfiber entangelment through needling or the like because of their small elongations and linear fiber morphology.
An object of the present invention is to provide a solution to such problems with production of a carbon fiber structure of optically anisotropic pitch that entanglement of a carbon fiber of optically anisotropic pitch through needling or the like is liable to be ineffective because of its comparatively poor twisting, bending and crimping properties, unlike those of common organic fibers, as well as because of its substantially circular cross section; and that the entanglement cannot go on smoothly because the carbon fiber of optically anisotromic pitch is liable to break sooner than move during the course of entanglement because of its very small elongation.
Another object of the present invention is to provide a carbon fiber structure of optically anisotropic pitch type which can be produced without the foregoing problems with the production thereof being involved and is increased in bulk density.